1. Field of the Invention
The present invention relates to a method and apparatus for nested automatic protection switching (nested APS) in which a communication line running over a plurality of sections is recovered using a series connection of protection lines provided in each section by communicating using a command and response defined in the overhead of each section, and more particularly to a method and apparatus for implementing nested APS by using the K1 and K2 bytes defined for APS in the Synchronous Optical Network (SONET) established by the American National Standards Institute (ANSI) or the Synchronous Digital Hierarchy (SDH) standardized in accordance with International Telecommunications Union-Telecommunications Standardization Sector (ITU-T) recommendations.
2. Description of the Related Art
Some communication paths established over physical lines may end at a certain node, and others may just pass through that node. When the capacity of a communication path just passing through a node is large, it is required that the physical lines be simply connected by a regenerator (REG) to reduce the cost, and also that the communication path be recovered using a series connection of protection lines provided in each section between opposing nodes without providing separate protection lines for such physical lines. The APS in which a communication line running over a plurality of sections is recovered using a series connection of protection lines provided in each section is generally known as nested APS.
In SONET and SDH, the K1 and K2 bytes are defined in the section overhead (SOH) for signaling for APS operations. Since these K1 and K2 bytes are transferred only within a section, if, for a communication line running over a plurality of sections, the contents of these bytes can be transmitted to an adjacent section, nested APS can be realized in SONET and SDH.
Japanese Unexamined Patent Publication No. 7-177114 proposes a technique for transmitting the contents of the K1 and K2 bytes to a node not directly opposing the originating node, by moving the contents of the K1 and K2 bytes in the SOH to the Z3 and Z4 bytes in the path overhead (POH).
However, some paths may end at that node and others may just pass through it; therefore, in order to carry the K1 and K2 bytes on the passing path without fail, the above technique would require checking which path is the passing path and performing POH processing for that path, but in practice this is very difficult.